The long term objective of this project is to explain how embryonic cells are committed to express certain sets of genes later in development. At present, relatively little is known at the molecular level about now these decisions are made. We have identified and cloned genes that may be involved in the process of embryonic determination in frogs. These include (1) genes that are expressed maternally and whose transcripts show the unusual property of being localized in eggs and (2) genes that contain homeoboxes, called Xhox. The Xhox genes are of interest because the homeobox is a conserved sequence that is found in Drosophila genes known to be important for specifying cell fates in flies. In addition, we propose to analyze the transcriptional regulation of a gene called GS17 because it is first transcribed precisely at the mid-blastula transition, the time at which the embryonic genome is activated. Together these genes provide us with the tools for a detailed study of the molecular biology of early determinative events in frog development. Our specific experimental objectives are: 1. To determine how maternal RNAs are localized to different regions of the frog egg. 2. To determine the function of the proteins translated from these localized mRNAs. 3. To continue developing anti-sense RNA and sense RNA injections using the SP6 system in order to probe gene function during early development. 4. To investigate the developmental expression and function of Xenopus homeobox genes. 5. To identify the cis and trans-acting signals involved in the selective activation of the embryonic genome. The medical significance of this work derives from the possibility that understanding how animals develop and how cells selectively express genes may be helpful for understanding certain diseases.